Fuel cell technology is a relatively recent development in the automotive industry. It has been found that fuel cell power plants are capable of achieving efficiencies as high as 55%. Furthermore, fuel cell power plants emit only heat and water as by-products.
A fuel cell includes three components at its core: a cathode catalyst layer, an anode catalyst layer, and an electrolyte that is sandwiched between the cathode and the anode layers and conducts protons. This three-layer sandwich as used in a proton-exchange membrane (PEM) fuel cell will be referred to herein as a membrane electrode assembly (MEA), and it is sometimes referred to as a catalyst-coated membrane (CCM). In operation, the catalyst in the anode layer splits hydrogen into electrons and protons. In a single fuel cell arrangement, the electrons are distributed as electric current from the anode, through an external circuit where they can provide electrical energy, and then to the cathode. The protons migrate from the anode through the electrolyte to the cathode. The catalyst in the cathode layer facilitates splitting of oxygen molecules and the subsequent reaction with the protons (passing through the membrane) and the electrons (returning from providing electrical energy) to form water. Individual fuel cells can be stacked together in series to generate increasingly larger voltages and quantities of electricity.
In a PEM fuel cell, a polymer membrane serves as the electrolyte between a cathode and an anode. The polymer membrane currently being used in fuel cell applications requires a certain level of humidity to facilitate proton conductivity of the membrane. Therefore, maintaining the proper level of humidity in the membrane, through humidity/water management, is very important for the proper functioning of the fuel cell. The polymer electrolyte membrane swells when absorbing water and shrinks when drying out thus demanding that the fuel cell stack be engineered to manage the effect of the changing membrane volume on stack size and internal compression.
Disposed outside of the MEA is a pair of gas diffusion media (to be described below) and conductive separator plates (also known as bipolar plates) for mechanically securing the MEA and electrically connecting adjacent MEAs in series in a fuel cell stack. Both sides of the separator plate, one of which is disposed toward the MEA and gas diffusion media of one cell and the other of which is disposed toward the MEA and gas diffusion media of the next cell in the stack, are provided with gas passages, also known as flowfields, for supplying reactant gases, hydrogen to the anode side of one MEA and air/oxygen to the cathode side of the adjacent MEA. The flowfields also provide a means by which product water can be removed from the cell, carried away by unreacted gases. The bipolar plate also normally contains coolant channels within it and is constructed so that the coolant is isolated from the gases fed to and removed from both the anode and the cathode.
In the fuel cell, a gas diffusion medium which is typically made from carbon fiber paper or carbon fiber cloth is interposed between the flowfield of the bipolar plate and the MEA to facilitate optimum diffusion of the reaction gases to the electrodes, provide optimum conduction of electrons, transfer heat generated at the MEA to the coolant within the bipolar plate coolant channels, and facilitate transport of product water from the cathode to the flowfield. The diffusion medium also acts as a mechanical buffer layer between the soft MEA and the stiff bipolar plates by accommodating thickness variations in both the bipolar plates and the diffusion media as well as protecting the MEA from being damaged by the bipolar plate upon compression. Diffusion media are typically sheet-like in geometry, about 100-400 microns thick and cover the entire active area of the cell (usually 50-1000 cm2). In the discussion below we refer to the “thickness direction” of the diffusion media (100-400 microns thick, normally 150-300 microns thick) as the z-direction. This differentiates it from the two “in-plane directions” of the sheet, referred to below as the x-y directions.
On the one hand, it is desirable for diffusion media to be stiff in the x-y directions such that upon compression, the diffusion media will not intrude into the flowfield channels of the bipolar plate. Such intrusion increases the pressure drop from the gas inlet of the respective flowfield to the outlet of the flowfield, which will increase the compressor capacity and electrical consumption requirements. In addition, it may cause a large pressure difference between anode and cathode compartments, thus damaging the MEA. In addition, stiffness of the diffusion medium in the x-y direction is desired in order to increase the contact pressure between the MEA and the diffusion medium over the flowfield channel region, thus reducing the electrical and thermal contact resistance between MEA and diffusion media. The stiffness of the diffusion media is defined as the force required to produce a defined deformation in the x or y direction [Timoshenko S. P. and Gere J. M., 1972, Mechanics of Materials, Litton Education Publishing, Inc.] It depends on the modulus of elasticity (an intrinsic material property) and the material thickness.
On the other hand, a compressible and elastic property in the z-direction of the diffusion medium is also advantageous. This reduces local high stress spots during compression of the stack. In addition, it maintains the contact between the MEA and the diffusion medium during the membrane swelling and shrinking cycles. Furthermore, a diffusion media with high compressibility in the z direction has the ability to compensate for thickness variations in the diffusion media and bipolar plates. Compressibility is defined as the compressive strain at a defined compressive load applied in the z direction, where compressive strain is defined as the ratio of compressive deformation to the original thickness. A diffusion media typically needs to exhibit compressive strains in the range of 10 to 50% when under stack compressive loads ranging from 50 to 400 psi. Thus, it is clear that the mechanical properties of the diffusion media must be optimized to meet the various requirements in the fuel cell stack. All of the desirable properties are difficult to achieve simultaneously. For example, achieving extremely anisotropic mechanical properties, stiffness (in the x-y directions) and sufficiently high compressibility (in the z direction), in the same material is a materials engineering challenge.
Tests commonly used to determine the mechanical properties of a gas diffusion medium material include bending tests and compressive stress-strain tests. In a bending test of a sheet-like material (e.g. ASTM D790 and ASTM D5934), the modulus of elasticity and modulus of rupture of the material in the x-y directions are measured. A high magnitude of the bending stiffness, as a result of high modulus of elasticity and/or thickness, increases compression of the diffusion media between the MEA and the bipolar plate over the plate flowfield channels, thus minimizing contact resistance there. Minimizing contact resistance over the channels is important to minimize voltage losses and achieve maximum fuel cell efficiency. In the compressive stress-strain test (e.g. ASTM E111), the material is compressed in the z-direction and strain is monitored as a function of stress.
In the manufacturing of gas diffusion medium materials for fuel cells, difficulty is encountered in fabricating a material that exhibits relatively high compressibility in the z-direction combined with stiffness in the x-y directions. For example, the wet-laid carbon fiber papers such as Toray TGPH-060 carbon fiber substrates are relatively stiff in the x-y directions due to the properties of carbon fibers and the impregnation of resin binder during the manufacture process. However, this type of wet-laid carbon fiber paper exhibits less compressibility than many commonly used diffusion media such as air-laid hydro-entangled carbon fiber paper (such as produced by Freudenberg, Germany) and woven carbon cloths (Zoltek, USA). Whereas these materials exhibit superior compressibility in the z-direction, they lack the desired stiffness in the x-y direction, resulting in higher contact resistance over the channel and higher channel intrusion.
Accordingly, a multi-layer diffusion medium substrate is needed which combines the properties of a compressible substrate with those of a stiff substrate to achieve the optimum diffusion medium material for use in a fuel cell.